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PIC III System Functions

CAPACITY CONTROL — Generally the chiller adjusts ca-

pacity in response to deviation of leaving or entering chilled

water temperature from CONTROL POINT. CONTROL

POINT is based on the configured SETPOINT (in the

SETPOINT screen: LCW SET POINT or ECW SET POINT or

ICE BUILD SET POINT), and CONTROL POINT is equal to

this SETPOINT plus any active chilled water reset value. A re-

set value may originate from any of the three chilled water/brine

reset options configured in the ICVC Service/Equipment

Service/TEMP_CTL screen (see page 49) or from a CCN de-

vice. The default reset value is 0° F. CONTROL POINT may be

viewed or manually overridden from the MAINSTAT screen.

Minor adjustments to the rate of capacity adjustment can be

made by changing PROPORTIONAL INC (Increase) BAND,

PROPORTIONAL DEC (Decrease) BAND, and PROPOR-

TIONAL ECW (Entering Chilled Water) GAIN in the Service/

Equipment Service/SETUP2 screen. Increasing the PROPOR-

TIONAL INC BAND or PROPORTIONAL DEC BAND, or

decreasing PROPORTIONAL ECW GAIN will reduce the

amplitude of the capacity control response (within limits). See

also Proportional Bands on page 41.

Factors and variables used in the capacity control determi-

nation are displayed in the Service/Control Algorithm Status/

Capacity screen and in the Status/COMPR screen. Viewing this

data will aid in troubleshooting and understanding current

operation.

Variable Speed (VFD) Application

— The PIC III controls

the machine capacity by modulating both motor speed and

inlet guide vanes in response to chilled water temperature devia-

tion from the CONTROL POINT (see above). During opera-

tion, when the CONTROL POINT is not met within 1/3 of the

width of the CHILLED WATER DEADBAND, the controller

will calculate a GUIDE VANE DELTA which will effect a per-

centage change to either the guide vane position or TARGET

VFD SPEED. Factors considered in the capacity control algo-

rithm include: (1) the sign and magnitude of GUIDE VANE

DELTA (based on deviation from CONTROL POINT adjusted

for the error trends and CHILLED WATER DEADBAND), (2)

ACTUAL GUIDE VANE POSITION, (3) ACTUAL VFD

SPEED, and (4) surge prevention mode. Generally the control-

ler will maintain the highest inlet guide vane setting at the low-

est speed to maximize efficiency while avoiding surge.

First, the calculation of GUIDE VANE DELTA is performed.

If GUIDE VANE DELTA is positive, the response will be an

IGV or VFD position increase (within limits). If GUIDE VANE

DELTA is negative, the response will be an IGV or VFD posi-

tion decrease (within limits). Next, the surge prevention mode is

determined based on location of the present operating point on

the CHILLED WATER DELTA T/ACTIVE DELTA P map rel-

ative the configured surge prevention line. This mode will either

be Normal or Surge Prevention. The following table indicates

which output is modulated first. When the first output reaches its

limit (ACTUAL GUIDE VANE position reaches maximum),

the second output is modulated. See Table 5.

Table 5 — Guide Vane Delta Modes

Normal Control mode occurs when ACTIVE DELTA T

> SURGE/HGBP DELTA T.

Surge Prevention Mode occurs when ACTIVE DELTA T

 SURGE/HGBP DELTA T.

The VFD GAIN parameter allows for additional adjustment

of the VFD response. Increasing VFD GAIN will increase the

rate of speed change.

Generally for the case of line voltage equaling motor volt-

age (460 volts), VFD output (motor) current is a few percent

higher than line current at full speed (60 Hz). As drive speeds

decrease from maximum, drive output voltage decreases

linearly with output frequency, and motor current continues to

increase relative to line current.

The TARGET VFD SPEED, ACTUAL VFD SPEED and the

VFD GAIN can be viewed and modified in the CAPACITY

display screen. The TARGET VFD SPEED can be manually

overridden by the operator from the COMPRESS screen. The

VFD MINIMUM SPEED, VFD MAXIMUM SPEED, VFD

GAIN and VFD INCREASE STEP can be selected and modi-

fied in the SETUP2 display screen. TARGET and ACTUAL

VFD SPEED can be viewed in the COMPRESS screen.

ECW CONTROL OPTION — If this option is enabled, the

PIC III uses the ENTERING CHILLED WATER temperature to

modulate the vanes instead of the LEAVING CHILLED

WATER temperature. The ECW CONTROL OPTION may be

viewed on the TEMP_CTL screen, which is accessed from the

EQUIPMENT SERVICE screen.

CONTROL POINT DEADBAND — This is the tolerance

range on the chilled water/brine temperature control point. If

the water temperature goes outside the CHILLED WATER

DEADBAND, the PIC III opens or closes the guide vanes until

the temperature is within tolerance. The PIC III may be config-

ured with a 0.5 to 2 F (0.3 to 1.1 C) deadband. CHILLED

WATER DEADBAND may be viewed or modified on the

SETUP1 screen, which is accessed from the EQUIPMENT

SERVICE table.

For example, a 1 F (0.6 C) deadband setting controls the

water temperature within ±0.5 F (0.3 C) of the control point.

This may cause frequent guide vane movement if the chilled

water load fluctuates frequently. A value of 1 F (0.6 C) is the

default setting.

A deadband is a span of measurement in which a controller

takes no action. In the PIC controls it is a temperature range

centered on the CONTROL POINT. If the LEAVING

CHILLED WATER TEMP falls within the CHILLED WATER

DEADBAND, the guide vanes will not move.

The purpose of the deadband is to prevent slight fluctua-

tions in ENTERING CHILLED WATER TEMPERATURE

from keeping the guide vanes in constant movement.

Adjusting The Deadband

— The default setting of the

CHILLED WATER DEADBAND, is 1°F, which is 0.5°F above

and below the CONTROL POINT. The CONTROL POINT set-

ting range is 0.5 to 2° F. If temperature control is satisfactory

and the guide vanes are stable, do not change the setting.

IMPORTANT: Words not part of paragraph headings and

printed in all capital letters can be viewed on the ICVC

(e.g., LOCAL, CCN, RUNNING, ALARM, etc.). Words

printed both in all capital letters and italics can also be

viewed on the ICVC and are parameters (CONTROL

MODE, TARGET GUIDE VANE POS, etc.) with associated

values (e.g., modes, temperatures, pressures, percentages,

on, off, enable, disable, etc.). Words printed in all capital let-

ters and in a box represent softkeys on the ICVC (e.g.,

and ). See Table 4 for examples of the type

of information that can appear on the ICVC screens. Figures

14-20 give an overview of ICVC operations and menus.

ENTER

EXIT

GUIDE VANE

DELTA

NORMAL

CONTROL

MODE

SURGE

PREVENTION

MODE

IGV VFD IGV VFD

From +0.2 to

+2.0

Increase

1st

Increase

when IGV

= max

Increase

only if VFD

speed =

max and if

hot gas

bypass is

present

and open

Increase

1st

From –0.2 to

–2.0

Decrease

when VFD

speed =

min

Decrease

1st

Decrease —

1 2 ... 35 36 37 38 39 40 41 42 43 44 45 ... 189 190

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Carrier XRV Spécifications Page 40

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